1. Field of the Invention
This invention relates to a pigment dispersed aqueous ink composition for use in ink jet printing which gives reliable printing performance and yields printed images which have excellent print quality.
2. Background Art
Ink jet printing is a non-impact printing process in which the printer produces droplets of ink in response to digital signals, such as those generated by a computer. The droplets of ink are deposited on a substrate such as paper or transparent films. Ink jet printers have found broad commercial acceptance due to their print quality, low cost, relatively quiet operation, and graphics capability. Thermal (bubble jet) and piezoelectric drop-on-demand printers have been especially successful in the marketplace and have found broad application as printers for personal computers in the office and the home.
The inks used in ink jet printers can be classified as either dye based inks or pigment based inks. Dye based inks are satisfactory for most applications, but generally have poor light fastness and water resistance. As a printed document is expected to have a certain degree of permanency, the lack of light fastness and water resistance of the printed image derived from dye based inks is a problem. Pigment based inks can be prepared which have excellent light fastness and water resistance. Thus, for purposes of obtaining a printed document with a reasonable degree of permanency, pigment based inks are preferred over dye based inks.
These major concerns dominate ink jet technology: (1) reliability, (2) drying rate, and (3) print quality. Typically, reliability is evaluated with respect to the following four criteria. The first is robustness with respect to continuous printing conditions such that the ink droplet weight does not change over time and good directionality is maintained. Good directionality means that the angular deviation of an ejected ink droplet from a nozzle is within about .+-.0.5.degree. from the normal to the plane of the nozzle. The second is robustness with respect to intermittent printing conditions such that the nozzles do not clog over the time interval in which printing is discontinued. The third is robustness with respect to long term storage of the ink within the print head such that printing behavior unchanged from the original printing behavior (ink droplet) weight and good directionality) can be restored after applying a limited amount of suction to the nozzles. The fourth is chemical and physical stability of the ink towards storage at two temperature extremes and towards cycling between those two temperature extremes for an extended period of time.
The drying rate of the ink is an important factor in determining the throughput rate of the printer. In a sheet fed printer, the ink on a printed sheet must be dry before the succeeding sheet contacts it. If the ink is not dry, smearing will occur.
Print quality is typically defined in terms of two general factors: (1) color properties and (2) non-color image characteristics. Color properties of the ink are measured by optical density and the color coordinates which determine the hue. Non-color characteristics which determine the definition of an image are resolution (drops per unit area), the area coverage per drop, edge acuity or sharpness, and the extent of peripheral defects such as satellites (stray droplets around the perimeter of a printed character) or feathering.
A great concern with ink jet printing is the level of print quality, as defined by edge acuity or sharpness of an image and minimal feathering, which can be obtained on "plain paper." The term "plain paper" refers to a broad set of commercial papers, especially those available for use in electrographic copying. Such commercial papers do not rely on a unique structure, composition, or narrow set of properties such that only an ink jet printer can make best use of the paper's properties. In recent years there has been an increasing demand for ink jet printers which provide excellent print quality on plain paper.
In terms of print quality on plain paper, suitably designed pigment based inks are preferred over both conventional pigment based inks and dye based inks. When a liquid ink droplet contacts the paper surface as a result of ink jet printing, the liquid spreads out from the impact origin and penetrates the paper. Cellulose fibers, present in most plain papers, tend to act as wicks which draw the liquid along the length of the individual fibers by capillary action.
In dye based inks, in which the colorant is homogeneously dissolved in the liquid, the colorant will spread out, penetrate, and be drawn along the length of cellulose fibers to the exact same degree as the liquid. The typical result for a dye based ink is a colored dot which has poorly defined feathered edges.
In pigment based inks, in which the colorant is homogeneously dispersed in the liquid, unless the dispersion stability of the colorant is disrupted upon contact with the paper, the colorant will spread out, penetrate, and be drawn along the length of cellulose fibers to the nearly the same degree as the liquid. The typical result for a conventional pigment based ink is a colored dot which has poorly defined feathered edges. In contrast, a suitably designed pigment based ink, in which the dispersion stability of the colorant is disrupted upon contact with the paper, the colorant will not spread out, penetrate, and be drawn along the length of cellulose fibers in the same way as the liquid. For this type of ink, the colorant effectively separates from the liquid carrier. The result is a colored dot which has a sharp edge boundary with negligible feathering.
Water-based pigment dispersions are well known in the art, and have been used commercially for applying films, such as paints, to various substrates. A stable pigment dispersion is obtained by using a pigment dispersant which provides stability through either steric stabilization only or a combination of both steric stabilization and ionic stabilization.
Examples of polymer dispersants which provide only steric stabilization belong to the group of non-ionic water soluble polymers: polyvinyl alcohol, cellulosics, ethylene oxide modified phenols, and ethylene oxide/propylene oxide polymers. Pigment dispersions which incorporate such polymers do not lose their dispersion stability upon contact with paper, and thus, yield printed images with poorly defined feathered edges.
Examples of polymer dispersants which provide only steric stabilization belong to the group on non-ionic water soluble polymers: polyvinyl alcohol, cellulosics, ethylene oxide modified phenols, and ethylene oxide/propylene oxide polymers. Pigment dispersions which incorporate such polymers do not lose their dispersion stability upon contact with paper, and thus, yield printed images with poorly defined feathered edges.
Examples of polymer dispersants which provide both steric and ionic stabilization are those constructed from monomers of neutralized acrylic, maleic, or vinyl sulfonic acid. Pigment dispersions, which incorporate suitably designed polymer dispersants of this type, can be made to lose their dispersion stability upon contact with paper and yield printed images with sharp edges boundaries and negligible feathering. The means by which polymer dispersants of this type lose their dispersion stability upon contact with paper is described below.
Because polymers of this type contain multiple neutralized acid functionalities, they can be classified as anionic polyelectrolytes. Typically, anionic polyelectrolytes bind to multivalent cations such as magnesium, calcium, and aluminum. The strength and sensitivity of the multivalent cation binding depend on the linear charge density and the structure of the polyelectrolyte. For suitably designed pigment dispersions containing polymers, which have multiple neutralized acid functionalities, the polymers will bind to multivalent cations which are present on the surface of typical plain papers. If the degree of multivalent cation binding is sufficient, the anionic charge density of the dispersant will be partially or completely neutralized. Charge neutralization results in the loss of ionic stabilization with the concomitant result that the dispersion stability of the colorant is disrupted. As noted above, such disruption of the dispersion stability of the colorant will result in the colorant separating from the liquid carrier. The net result is a printed image which has sharp edge boundaries with negligible feathering.
The strength and sensitivity of polyelectrolyte multivalent cation binding depend on both the linear charge density and structure of the polyelectrolyte. In general, the higher the linear charge density of the polyelectrolyte, the greater the binding interaction between the polyelectrolyte and multivalent cations. Linear charge density is higher for polymers, in which the neutralized acid functionalities are on adjacent monomer units, than for polymers, in which the neutralized acid functionalities are on monomer units which are interspersed with nonionic monomer units. As an example, polyacrylic acid has a higher linear charge density than a random polymer of acrylic acid and styrene. The structure of the polyelectrolyte also can influence the strength and sensitivity of multivalent cation binding. If the binding sites on the polyelectrolyte have shapes and coordination environments which optimally match the ionic radii of multivalent cations, the binding interaction between that polyelectrolyte and multivalent cations will be greater that that of a similar polyelectrolyte with no special structural features. Obviously, a binding site which is optimal for Ca.sup.2+ (ionic radius: 1.14 .ANG.; six-coordinate) will not necessarily be optimal for Mg.sup.2+ (ionic radius: 0.86 .ANG.; six-coordinate). It also follows that a binding site which is optimal for Ca.sup.2+ (ionic radius: 1.14 .ANG., six coordinate) may also bind reasonably well to monovalent Na.sup.+ (ionic radius: 1.16 .ANG.; six-coordinate).
It must be noted, however, that polymer dispersants which are constructed from only monomers of neutralized acrylic, maleic, or vinyl sulfonic acids, which are hydrophilic, do not yield stable pigment dispersions. This is because hydrophilic functionalities do not adhere sufficiently to the surface of conventional pigments. Stable pigment dispersions can be obtained only if the polymer dispersant also contain one or more hydrophobic segments which adsorb onto and adhere to the surface of the pigments. Thus, polymer dispersants, which provide both steric and ionic stabilization, must contain both hydrophobic segments and hydrophilic segments.
In U.S. Pat. No. 4,597,794, an aqueous ink dispersion for ink jet printers is described in which the pigment is dispersed using a random copolymer having hydrophilic neutralized carboxylic acid substituents and hydrophobic aromatic ring substituents that adhere to the pigment surface. Because the polymer is a random copolymer, the linear charge density is low and sensitivity towards multivalent cations on the surface of typical plain papers is expected to be low.
In U.S. Pat. No. 5,085,698, an aqueous ink dispersion for ink jet printers is described in which the pigment is dispersed using an AB or BAB block copolymer. The A segment of the block copolymer is a hydrophobic, water insoluble homopolymer or copolymer of an acrylic acid ester or an acrylic acid amide. The B segment of the block copolymer is a hydrophilic water soluble homopolymer or copolymer of an acrylic acid, an acrylic acid amide, or an acrylic acid ester in which the alcohol portion of the ester contains hydrophilic functionalities. For block polymers with B segments which contain acrylic acid functionalities, the acid functionalities are neutralized with neutralizing agents selected from the group consisting of organic bases, alkanolamines, alkali metal hydroxides, and mixtures thereof. The specific examples and comparative examples given in this patent show that inks formulated with AB or BAB block copolymer pigment dispersants, having discrete hydrophobic and hydrophilic segments, have better printhead performance than inks formulated with random copolymers of hydrophobic and hydrophilic monomers. Furthermore, because block copolymers have a high linear charge density on the hydrophilic B segment, better sensitivity than the above described random copolymers is expected towards multivalent cations on the surface of typical plain papers. Although the hydrophilic B segments have a high linear charge density, they do not have any special structural features which favor binding to multivalent cations.
As can be seen from the above examples, there remains a need for improved pigment dispersed aqueous ink compositions in which the pigment is stabilized by polymer dispersants which contain both hydrophobic and hydrophilic segments. Improved pigment dispersed aqueous ink compositions which give reliable printing performance and yield printed images which have excellent print quality are desired to meet the demanding needs of commercial ink jet printers.